The present invention relates generally to uniform ionization of gaseous media useful in pulsed gas lasers, and more particularly to a three-electrode, high gain, high pressure CO.sub.2 laser which allows the use of dc voltage across the gain medium, thereby eliminating the need for a costly and less reliable high-energy pulsed discharge system for its excitation.
Two journal articles describe double discharge CO.sub.2 lasers using three electrodes.
1. In "Double Discharge Excitation for Atmospheric Pressure CO.sub.2 Lasers" by Albert K. LaFlamme, Rev, Sci. Instr. 41, 1578 (1970), a modified, double discharge laser is discussed. A single trigger device controls both the preionization and the gain medium pumping discharges. Although the apparatus of the subject invention also has one voltage controlling switch, this switch does not carry the substantial main laser discharge current as is the situation for the LaFlamme switch. This results in a significant improvement in laser cost, reliability and pulse repetition rate over lasers which require high voltage, high current rapid switching devices.
2. In "A 300-J Multigigawatt CO.sub.2 Laser" by Martin C. Richardson, A. J. Alcock, Kurt Leopold, and Peter Burtyn, IEEE J. Quantum Electron. 9, 236 (1973), a true double discharge laser is described. A first trigger device fires the preionization discharge, while a second such device prevents the main discharge until sufficient ionization has occurred in the region of the anode. Here, as in Ref. 1, one trigger unit is carrying the entire main discharge current. As described hereinabove, such devices are expensive, have a tendency to be unreliable, and have limited rates of repetition.
A single patent, U.S. Pat. No. 4,412,333, "Three-Electrode Low Pressure Discharge Apparatus and Method for Uniform Ionization of Gaseous Media" issued to Edward J. McLellan on Oct. 25, 1983, which is hereby incorporated by reference herein, teaches the use of rapid electric discharge preionization to initiate and control the main discharge in the gaseous medium. The apparatus described therein avoids the use of a high voltage switch to hold the high energy discharge back until sufficient ionization has occurred to permit the main discharge to occur without serious arcing. However, the highest pressure which this device was found to operate without arcing was about 80 torr with mixtures of CO.sub.2, N.sub.2, and He, thereby limiting the available energy output from such a device. The apparatus and method of McLellan's invention also operates at lower voltage applied to the main discharge than similar devices.